Heterogeneous catalyst

ABSTRACT

A heterogeneous catalyst comprising a support and a noble metal, wherein said support comprises silicon, and wherein said catalyst comprises from 0.1 to 40 mol % titanium and from 0.1 to 10 mol % of at least one noble metal.

BACKGROUND OF THE INVENTION

The invention relates to a heterogeneous catalyst. The catalyst isespecially useful in a process for preparing methyl methacrylate frommethacrolein and methanol.

Heterogeneous catalysts having noble metals supported on silica incombination with alumina and other elements are known, see e.g. U.S. PatNo. 8,461,373. However, there is a need for additional catalystparticles with improved properties.

SUMMARY OF THE INVENTION

The present invention is directed to a heterogeneous catalyst comprisinga support and a noble metal, wherein said support comprises silicon, andwherein said catalyst comprises from 0.1 to 40 mol % titanium and from0.1 to 10 mol % of at least one noble metal, wherein mole percentagesare based on total moles of silicon atoms and metal atoms.

The present invention is further directed to a method for preparation ofa heterogeneous catalyst comprising a support and a noble metal, whereinsaid support comprises silicon and titanium, and wherein said catalystcomprises from 0.1 to 40 mol % titanium and from 0.1 to 10 mol % of atleast one noble metal, wherein mole percentages are based on total molesof silicon atoms and metal atoms; said method comprising contacting asupport comprising silicon with a titanium salt and a noble metal salt.

The present invention is further directed to a catalyst bed comprisingthe catalyst.

DETAILED DESCRIPTION OF THE INVENTION

All percentage compositions are weight percentages (wt %), and alltemperatures are in ° C., unless otherwise indicated. A “noble metal” isany of gold, platinum, iridium, osmium, silver, palladium, rhodium andruthenium. More than one noble metal may be present in the catalyst, inwhich case the limits apply to the total of all noble metals. A “metal”is an element in groups 1 through 12 of the periodic table, excludinghydrogen, plus aluminum, gallium, indium, thallium, tin, lead andbismuth. The “catalyst center” is the centroid of the catalyst particle,i.e., the mean position of all points in all coordinate directions. Adiameter is any linear dimension passing through the catalyst center andthe average diameter is the arithmetic mean of all possible diameters.The aspect ratio is the ratio of the longest to the shortest diameters.

Preferably, the support is a particle comprising a refractory oxide;preferably silica, titania, magnesia, or a combination thereof;preferably the support is silica or silica modified with otherrefractory oxides. Preferably the support has a surface area greaterthan 10 m²/g, preferably greater than 30 m²/g, preferably greater than50 m²/g, preferably greater than 100 m²/g, preferably greater than 120m²/g. Preferably, the support comprises a silica particle comprisingfrom 0.1 to 40 mol % titanium, based on total moles of silicon atoms andmetal atoms (i.e., excluding oxygen and other non-metallic elementsother than silicon), preferably at least 0.1 mol %, preferably at least1 mol %; preferably no more than 40 mol %, preferably no more than 30mol %. Preferably, the support comprises no more than 10 mol % aluminum,based on total moles of silicon atoms and metal atoms, preferably nomore than 5 mol %, preferably no more than 2 mol %, preferably no morethan 1 mol %, preferably no more than 0.5 mol %.

Preferably, the aspect ratio of the catalyst particle is no more than10:1, preferably no more than 5:1, preferably no more than 3:1,preferably no more than 2:1, preferably no more than 1.5:1, preferablyno more than 1.1:1. Preferred shapes for the particle include spheres,cylinders, rectangular solids, rings, multi-lobed shapes (e.g.,cloverleaf cross section), shapes having multiple holes and “wagonwheels,” preferably spheres. Irregular shapes may also be used.

Preferably, the catalyst comprises 0.1 to 10 mol % of at least one noblemetal, 50 to 95 mol % Si, 0.1 to 40 mol % Ti and 0.1 to 40 mol % alkalimetal or alkaline earth metal or a combination thereof, based on totalmoles of silicon atoms and metal atoms. Preferably, the catalystcomprises at least 55 mol % Si, preferably at least 60 mol %, preferablyat least 65 mol %, preferably at least 70 mol %; preferably no more than97 mol %. Preferably, the catalyst comprises at least 0.1 mol % Ti,preferably at least 1 mol %, preferably at least 5 mol %; preferably nomore than 30 mol %, preferably no more than 20 mol %, preferably no morethan 15 mol %. Preferably, the catalyst comprises at least 0.1 mol %noble metal(s), preferably at least 0.2 mol %, preferably at least 0.3mol %; preferably no more than 7 mol %, preferably no more than 5 mol %,preferably no more than 3 mol %. Preferably, the catalyst comprises atleast 0.1 mol % alkali or alkaline earth metal(s), preferably at least 1mol %, preferably at least 2 mol %; preferably no more than 30 mol %,preferably no more than 20 mol %, preferably no more than 15 mol %. Inone preferred embodiment of the invention, the catalyst comprises nomore than 20 mol % magnesium, based on total moles of silicon atoms andmetal atoms, preferably no more than 10 mol %, preferably no more than 5mol %, preferably no more than 2 mol %, preferably no more than 1 mol %.In one preferred embodiment of the invention, the catalyst comprises nomore than 20 mol % alkaline earth metals, based on total moles ofsilicon atoms and metal atoms, preferably no more than 10 mol %,preferably no more than 5 mol %, preferably no more than 2 mol %,preferably no more than 1 mol %.

Preferably, at least 90 wt % of the noble metal(s) is in the outer 80%of catalyst volume (i.e., the volume of an average catalyst particle),preferably the outer 60%, preferably in the outer 50%, preferably in theouter 40%, preferably the outer 30%, preferably the outer 25%.Preferably, the outer volume of any particle shape is calculated for avolume having a constant distance from its inner surface to its outersurface (the surface of the particle), measured along a lineperpendicular to the outer surface. For example, for a sphericalparticle the outer x % of volume is a spherical shell whose outersurface is the surface of the particle and whose volume is x % of thevolume of the entire sphere. Preferably, at least 95 wt % of the noblemetal is in the outer volume of the catalyst, preferably at least 97 wt%, preferably at least 99 wt %. Preferably, at least 90 wt % (preferablyat least 95 wt %, preferably at least 97 wt %, preferably at least 99 wt%) of the noble metal(s) is within a distance from the surface that isno more than 15% of the catalyst diameter, preferably no more than 10%,preferably no more than 8%, preferably no more than 6%. Distance fromthe surface is measured along a line which is perpendicular to thesurface.

Preferably, the noble metal is gold or palladium, preferably gold.

Preferably, the average diameter of the catalyst particle is at least 60microns, preferably at least 100 microns, preferably at least 200microns, preferably at least 300 microns, preferably at least 400microns, preferably at least 500 microns, preferably at least 600microns, preferably at least 700 microns, preferably at least 800microns; preferably no more than 30 mm, preferably no more than 20 mm,preferably no more than 10 mm, preferably no more than 5 mm, preferablyno more than 3 mm. The average diameter of the support and the averagediameter of the final catalyst particle are not significantly different.

Preferably, the amount of noble metal as a percentage of the noble metaland the support is from 0.2 to 5 wt %, preferably at least 0.5 wt %,preferably at least 0.8 wt %, preferably at least 1 wt %, preferably atleast 1.2 wt %; preferably no more than 4 wt %, preferably no more than3 wt %, preferably no more than 2.5 wt %.

The catalyst of this invention is useful in a process for producingmethyl methacrylate (MMA) which comprises treating methacrolein withmethanol in an oxidative esterification reactor (OER) containing acatalyst bed. The catalyst bed comprises the catalyst particles and issituated within the OER that fluid flow may occur through the catalystbed. The catalyst particles in the catalyst bed typically are held inplace by solid walls and by screens. In some configurations, the screensare on opposite ends of the catalyst bed and the solid walls are on theside(s), although in some configurations the catalyst bed may beenclosed entirely by screens. Preferred shapes for the catalyst bedinclude a cylinder, a rectangular solid and a cylindrical shell;preferably a cylinder. The OER further comprises a liquid phasecomprising methacrolein, methanol and MMA and a gaseous phase comprisingoxygen. The liquid phase may further comprise byproducts, e.g.,methacrolein dimethyl acetal (MDA) and methyl isobutyrate (MIB).Preferably, the liquid phase is at a temperature from 40 to 120° C.;preferably at least 50° C., preferably at least 60° C.; preferably nomore than 110° C., preferably no more than 100° C. Preferably, thecatalyst bed is at a pressure from 0 to 2000 psig (101.3 to 13890.8kPa); preferably no more than 2000 kPa, preferably no more than 1500kPa. Preferably, pH in the catalyst bed is from 4 to 10; preferably atleast 4.5, preferably at least 5; preferably no greater than 9,preferably no greater than 8, preferably no greater than 7.5, preferablyno greater than 7, preferably no greater than 6.5. Preferably, thecatalyst bed is in a tubular continuous reactor.

Preferably, the catalyst is produced by precipitating on a supportparticle (preferably silica) titanium from a titanium salt and then thenoble metal from an aqueous solution of metal salts in the presence ofthe support. Preferred titanium salts include titanium acetate, titaniumsulfate, titanium(IV) oxysulfate, titanium chloride, titaniumoxychloride, titanium(IV) bis(ammonium lactato)dihydroxide solution,titanium(IV) 2-ethylhexyloxide, titanium(IV) butoxide, titanium (IV)isopropoxide and titanium(IV) oxyacetylacetonate. Preferred noble metalsalts include tetrachloroauric acid, sodium aurothiosulfate, sodiumaurothiomalate, gold hydroxide, palladium nitrate, palladium chlorideand palladium acetate. In one preferred embodiment, a titanium-modifiedsupport is produced by an incipient wetness technique in which anaqueous solution of a titanium precursor salt is added to a porousinorganic oxide such that the pores are filled with the solution and thewater is then removed by drying. Preferably, the resulting material isthen treated by calcination, reduction, or other treatments known tothose skilled in the art to decompose the titanium salts into metals ormetal oxides. Preferably, noble metal(s) is added to the calcinedtitanium-modified support by incipient wetness, followed by drying, andpreferably by calcination.

Calcinations preferably are carried out at a temperature from 250° C. to600° C.; preferably at least 300° C., preferably no more than 550° C.Preferably, the temperature is increased in a stepwise or continuousfashion to the ultimate calcination temperature.

In another preferred embodiment, the catalyst is produced by depositionprecipitation in which a porous inorganic oxide is immersed in anaqueous solution containing a suitable noble metal precursor salt andthat salt is then made to interact with the surface of the inorganicoxide by adjusting the pH of the solution. The resulting treated solidis then recovered (e.g. by filtration) and then converted into afinished catalyst by calcination, reduction, or other treatments knownto those skilled in the art to decompose the noble metal salts intometals or metal oxides.

EXAMPLES Example #1

Single Pass Fixed Bed Bubble Column Reactor Operation:

A feed consisting of 20 wt % methacrolein, 200 ppm inhibitor, and abalance of methanol was fed at a rate of 40 g/hr to a ⅜″ (9.5 mm)stainless steel tubular reactor containing a short front section ofborosilicate glass beads followed by 5 g of catalyst. Catalyst #1 wasutilized. A gas containing 8% oxygen in nitrogen was also feed to thereactor at a rate sufficient to obtain 4.5% O₂ in the vent. The reactorwas operated at 60° C. and 160 psig (1200 kPa). The product of thereactor was sent to a liquid-vapor separator and the vapor was sent to acondenser with liquid return and non-condensable gases going to thevent. Results are described in the below table.

Catalyst #1 Preparation:

Catalyst #1 was prepared by the incipient wetness technique using 20 gof Fuji Silysia Chemical, Ltd. CARiACT Q-10 support as the startingmaterial and adding titanium to the support material. Specifically 10.5g of titanium isopropoxide along with 3 g of glacial acetic acid wereadded to the catalyst in rotating equipment to ensure even distributionof the solution to the support material. The solution was at 40° C. whenadded. The modified support material was then dried under slight vacuumat 60° C. for 4 hrs and then calcined in air at ambient pressure byramping the temperature at 5° C. per minute from ambient to 125° C.,held for 1 hr and then ramped at 5° C. per minute up to 250° C. and heldfor 1 hr, then ramped at 5° C. per minute to 350° C. and held for 1 hrand finally ramped at 5° C. per minute to 450° C. and held for 4 hrs.Gold was then added to the support by incipient wetness techniqueutilizing 0.83 g of sodium aurothiosulfate in 10 g of deionized water at40° C. The resulting catalyst was dried and calcined in air using thesame heating profile as above. Analysis with a scanning electronmicroscope (SEM) equipped with energy-dispersive spectroscopy (EDS) ofthe catalyst clearly indicates that an eggshell deposition of both Tiand Au exists with the Au preferentially located only where Ti wasdeposited. The Ti and Au eggshell thickness was found to beapproximately 50 microns or less. With an estimated loading of 10 mol %in the outer 50 microns of the 1 mm diameter catalyst, the local loadingof titanium is estimated as up to 40 mol % as Ti/(Ti+Si).

Example #2 (Comparative)

Batch Recycle Fixed Bed Bubble Column Reactor Operation:

A feed solution of 150 g was prepared comprising 10 wt % methacrolein,200 ppm inhibitor and a balance of methanol, and placed in a 300 mlPARR® reactor which served as a gas disengagement vessel. The vesselliquid was maintained at a temperature of approximately 20° C. Theliquid feed was pumped at 7 mL/min from the gas-disengagement vesselinto the bottom of the vertically-oriented fixed bed reactor. Air andnitrogen gas was mixed to obtain 7.8 mol % oxygen and mixed with theliquid feed prior to entering the fixed bed reactor. The fixed bedreactor was a jacketed ¼″ (6.4 mm) stainless steel tube maintained at60° C. using an external heater. The reactor itself was packed with 2 mmglass beads to fill approximately 18 inches (46 cm) of the tube, thencatalyst. The remaining void at the top of the reactor was filled with 3mm glass beads. Liquid and gas exiting the top of the reactor were sentto a condenser and non-condensable gases were vented, while the liquidwas recycled back into the gas-disengagement vessel. Catalyst #2, aswell as the catalysts from examples #3, #4, and #5 below were run inthis manner

Catalyst #2 Preparation:

Catalyst #2 was prepared by incipient wetness of 4.1 g sodium goldthiosulfate dissolved in 100 g of water to make an aqueous solution andthen placed on 100 g of Fuji Silysia Chemical, Ltd. CARiACT Q-20 silicasupport material. The sample was dried at 120° C. for 1 hr followed bycalcination at 400° C. for 4 hr.

Example #3

Catalyst #3 Preparation:

Catalyst #3 was prepared by the following steps. First, a titaniumprecursor stock solution consisting of 51.7 g of titanium isopropoxideand 28.5 g glacial acetic acid was mixed and stirred at ambienttemperature. A support material was then prepared by impregnating 27.9 gof the above mentioned titanium stock solution to the incipient wetnesspoint of 20 g of Fuji Silysia Chemical, Ltd. CARiACT Q-10 silica supportmaterial. The sample was then dried at 125° C. for 1 hr, followed bycalcination at 250° C. for 1 hr, 350° C. for 1 hr, and 450° C. forovernight with a ramping rate of 5° C. per minute between differenttemperature settings. Gold deposition was achieved by impregnating asolution containing 0.4 g of sodium gold thiosulfate and 16 g ofdeionized water to 10 g of the above described support material to itsincipient wetness point. The sample was then dried at 120° C. for 1 hrfollowed by calcination at 400° C. for 4 hr. Analysis with a scanningelectron microscope (SEM) equipped with energy-dispersive spectroscopy(EDS) of the catalyst clearly indicates that an eggshell deposition ofboth Ti and Au exists with the Au preferentially located only where Tiwas deposited. The Ti and Au eggshell thickness was found to beapproximately 300 microns or less.

Example #4

Catalyst #4 Preparation:

Catalyst #4 was prepared by the following steps. First, a supportmaterial was prepared by impregnating titanium isopropoxide to theincipient wetness point of 10 g of Fuji Silysia Chemical, Ltd. CARiACTQ-10 silica support material. The sample was then dried at 125° C. for 1hr, followed by calcination at 250° C. for 1 hr, 350° C. for 1 hr, 450°C. for 1 hr and 550° C. for 12 hrs with a ramping rate of 5° C. perminute between different temperature settings. Gold deposition wasachieved by impregnating a solution containing 0.25 g of sodium goldthiosulfate and 9 g of deionized water to the incipient wetness point of6 g of the above described support material. The sample was then driedat 120° C. for 1 hr followed by calcination at 400° C. for 4 hrs.

Example #5

Catalyst #5 Preparation:

Catalyst #5 was prepared by the following steps. First, a supportmaterial was prepared by impregnating magnesium nitrate hexahydrate tothe incipient wetness point of 10 g of Fuji Silysia Chemical, Ltd.CARiACT Q-10 silica support material. The sample was then dried at 120°C. for 1 hr, followed by calcination at 450° C. for 4 hrs with a rampingrate of 5° C. per minute between different temperature settings. Aquantity of 8.5 g of titanium isopropoxide and 1.5 g of acetic acid weremixed to provide a titanium precursor solution and 3.1 g of the titaniumprecursor solution was then impregnated to the above mentioned calcinedMg-SiO₂. The sample was then dried at 120° C. for 1 hr, followed bycalcination at 550° C. for 6 hrs with a ramping rate of 5° C. per minutebetween different temperature settings. Gold deposition was achieved byimpregnating a solution containing 0.3 g of sodium gold thiosulfate and8 g of deionized water to the incipient wetness point of 8 g of theabove described support material. The sample was then dried at 120° C.for 1 hr followed by calcination at 400° C. for 4 hrs. The resultingsample contained a total of 4.7 wt % Mg and 4 wt % Ti on Si with 1.5 wt% Au loaded on that material. The sample was not assessed to determineif eggshell deposition existed.

Catalyst Performance:

Normalized Catalyst Catalyst Reactor STY MIB MMA Catalyst # DescriptionLoad (g) Type (mol/kg-hr) (ppm) Selectivity (%) 1 Au/Ti—SiO₂ 5 Single4.9 225 98.4 Pass 1 Au/Ti—SiO₂ 1 Batch 4.6 130 98.4 2 Au/SiO₂ 2 Batch1.75 400 99.1 (comp.) 3 Au/Ti—SiO₂ 1 Batch 3.3 160 94.8 4 Au/Ti—SiO₂ 1Batch 3.4 140 98.9 5 Au/Ti—Mg—SiO₂ 1 Batch 5.5 675 98.9 * The normalizedMMA selectivity is the percent MMA among products originating asmethacrolein reactant. MIB is reported in ppm by weight on a 100% MMAproduct basis.Crush Strength:

The mechanical strength of catalyst or catalyst support particles wasdirectly measured by crushing the particles to the point of mechanicalfailure. Crush strength testing was carried out using a Mecmesin M100EC.A single particle was placed on the platform and the top plunger wasallowed to press on the particle until the load reached a peak value andthe material failed. The peak load was recorded using a Shimpo FGE-100Xgauge. The test was repeated on 25 individual particles to obtain astatistical average of the crush strength for any given material.Results are tabulated below.

Crush Crush Material Diameter force Strength Catalyst # Description (mm)(Newton) (N/mm) na Q-10 2.6 51 20 2 (comp.) Au/SiO₂ 3.3 40 12 3Au/Ti—SiO₂ 3.2 60 19 5 Au/Ti—Mg—SiO₂ 3.2 24  8

The invention claimed is:
 1. A heterogeneous catalyst comprising asupport and a noble metal, wherein said support comprises silicon, andwherein said catalyst comprises from 0.1 to 40 mol % titanium and from0.1 to 10 mol % of at least one noble metal, wherein mole percentagesare based on total moles of silicon atoms and metal atoms, wherein theamount of noble metal as a percentage of the noble metal and the supportis from 0.2 to 2.5 wt %.
 2. The catalyst of claim 1 in which the noblemetal is selected from the group consisting of gold, palladium andcombinations thereof.
 3. The catalyst of claim 2 in which the catalysthas an average diameter from 60 microns to 10 mm.
 4. The catalyst ofclaim 3 in which the catalyst comprises 0.1 to 8 mol % of at least onenoble metal, 60 to 95 mol % silicon, 0.1 to 20 mol % titanium and 0.1 to20 mol % alkali metal or alkaline earth metal.
 5. The catalyst of claim4 in which the support is silica.
 6. The catalyst of claim 5 in whichthe noble metal is gold.